This invention relates to the treatment of aluminum-lithium alloys prior to heat treatment thereof and is particularly directed to a method of preventing lithium oxide formation in aluminum-lithium alloys during high temperature processing, e.g., solution heat treating, by application of a metal plate which can be readily removed following such processing.
When the recently developed aluminum-lithium alloys are processed at high temperature, such as at the solution heat treatment temperature or superplastic forming temperatures, lithium loss occurs, resulting in attendant alloy loss and associated property degradation. This is due to the propensity of Al-Li alloys to oxidize at high temperature, as noted in the article, "Effects of Lithium Loss on Strength and Formability of Aluminum-Lithium Alloys 8090 and 8091", J. M. Papazian, et al, Materials, Science and Engineering, 94 (1987), 219-224. Thus, the high temperature processing of any of the advanced Al-Li alloys will result in (1) lithium loss, due to formation of lithium oxide with the amount of lithium loss being dependent on time and temperature, (2) the lithium loss will result in an attendant loss in strength, particularly tensile strength of the alloy, and (3) the loss in strength will be a function of sheet gage, with the thinner gages being affected the most. At the solution heat treatment temperature, typically carried out at a temperature in the range of about 900.degree. to about 1,000.degree. F., a substantial loss of the lithium content of the alloy may occur, particularly with thin section material, within normal heat treatment times due to reactivity of lithium with the furnace atmosphere. Also, lithium oxide/hydroxide poses health concerns relative to breathing the oxide, which has been identified as an irritant.
Nevertheless, aluminum-lithium alloys are now of considerable interest for the production of structural components having a high strength/weight ratio.
U.S. Pat. No. 4,534,807 discloses heat treatment of an aluminum-lithium alloy being carried out in an atmosphere of carbon dioxide and water vapor, the partial pressure of the water vapor in such atmosphere being at least 4 torr. The patentee states that this treatment is effective in reducing oxidation of lithium in heat treatments carried out at temperatures in excess of 450.degree. C.
U.S. Pat. No. 3,404,998 discloses a method of metal plating aluminum alloys without permanent loss of strength or hardness due to annealing caused by high plating temperatures. The method comprises decomposition of a bis (arene) metal compound, the metal being vanadium, niobium, tantalum, chromium, molybdenum or tungsten, to form a metal plate on the aluminum alloy, followed by solution heat treatment and precipitation hardening to restore the strength and hardness to substantially their original levels.
U.S. Pat. No. 4,654,091 discloses protection of the free surfaces of a nickel superalloy article from oxidation and subsequent quench cracking during heat treatment by the application of a thin layer of nickel plate. The patentee states in column 1 of the patent that in the carburizing of steel, it is known to use localized copper plating to eliminate carburizing of selected surface portions. The patentee further states, however, that copper is an element known to adversely affect superalloys and, hence, is apparently undesirable for use therewith.
An object of the present invention is to provide a process for heat treatment of aluminum-lithium alloys while avoiding oxidation of lithium to lithium oxide.
Another object of the invention is the provision of relatively simple procedure for the treatment of aluminum-lithium alloys prior to high temperature heat treatment thereof, to avoid lithium loss and degradation of the mechanical properties of the aluminum-lithium alloy, e.g., tensile strength, resulting from such lithium loss.
A particular object of the present invention is to provide procedure to prevent oxidation of lithium in aluminum-lithium alloys during high temperature treatment thereof, by the application of a surface barrier on the aluminum-lithium alloy parts prior to heat treatment, which prevents such oxidation and attendant degradation of the properties of such aluminum-lithium alloys, followed by simple removal of the surface barrier after heat treatment.